The “lag time” of a smoke detector is commonly defined as the length of time from when a threshold smoke density is reached outside the detector to when the smoke detector responds. In the event of a fire, the lag time takes away from the Available Safe Egress Time (ASET), the time available for occupants to safely evacuate a building before the fire renders evacuation impossible. Thus, reducing the lag time in a smoke detector is critically important, because it can make the difference between life and death.
For ionization and photoelectric smoke detectors, the lag time can be substantial because both types require the use of an internal sensing chamber to physically isolate the smoke-detecting element from the ambient environment. The sensing chamber is typically enclosed by a baffle that restricts the flow of smoke into the chamber, thereby delaying the buildup of smoke compared to the smoke level outside the detector. Ionization and photoelectric smoke detectors can be combined with detectors sensitive to other products of a fire, such as carbon monoxide (CO) and heat, to lower the threshold smoke density at which the smoke detector responds. However, this approach does not remove the restriction on the flow of smoke into the sensing chamber, and significant time for smoke detector response is still needed.
Another shortcoming of ionization and photoelectric smoke detectors is that, in models equipped with a test feature, the testing mechanism tests the electrical circuitry only and not the operation of the smoke-detecting element. Specifically, activating the test mechanism does not introduce any smoke or smoke analog into the sensing chamber to test the response of the smoke detector. Thus, there is no assurance during a smoke detector test that the smoke-detecting element has sensitivity to the presence of smoke.
Accordingly, there is a need for detectors for smoke and/or gas, and related detection techniques, which decrease lag time and enable testing of the smoke-detecting element.